Modification of a unique supramolecular assembly of a pyridylphosphine ligand and a metal centered porphyrin complex is shown to give unprecedented selectivities to branched aldehydes via rhodium catalyzed hydroformylation of propylene and 1-octene. Use of magnesium in the porphyrin center provides the highest reported concentrations of iso-butyraldehyde and 2-methyl-octanal.